1. Field of Invention
The present invention relates to an optical amplifier. More particularly, the present invention relates to an optical amplifier of Michelson interferometer.
2. Description of Related Art
Typically, an optical fiber amplifier uses an erbium doped fiber (EDF) or other active fiber as a gain media. A semiconductor laser light source is also used to excite electrons from a lower energy level to a higher energy level as a pumping function. As an initial optical signal passes through the optical amplifier, it triggers the excited electrons to drop to its lower energy level through radiate photons, resulting in a stimulated emission. The optical signal therefore is amplified.
However, as the optical signal is amplified, a spontaneous emission, resulting in noise, does also simultaneously occur. The noise is also amplified by the optical amplifier to form an amplified spontaneous emission (ASE) noise and propagates along with the optical signal. The ASE noise not only reduces the light pumping efficiency, but also reduce a signal-to-noise ratio (SNR). As a result, the performance of the optical amplifier is degraded, and the communication quality is then deteriorated. It becomes a key point necessarily to be considered during designing the optical amplifier to suppress the ASE noise and increase an optical gain.
A U.S. Pat. No. 5,233,463 by Pirelli S.P.A. company (Italy) disclosed an optical amplifier using a two-level structure. Between the preamplifier and the post-amplifier, an optical isolator is added so as to isolate the ASE noise from the post-amplifier propagating in an opposite direction to the preamplifier. The preamplifier therefore can have greater optical gain, resulting in a better performance of the whole optical amplifier. However, the ASE noise originated from the preamplifier itself, which also includes optical fiber, is also amplified in the post amplifier. The performance in this manner is not at the best mode.
A U.S. Pat. No. 5,280,383 by AT&T Bell Lab. Disclosed another optical amplifier, which adds a filter after the isolator so as to filter away the ASE noise with a wavelength other than the desired wavelength used in communication. The filters only allows the light having the desired wavelength to pass and enter the post amplifier. Even though this manner can reduce the ASE noise, it also causes a large amount of insertion loss (IL). The transmission quality of the optical signal is greatly affected. Moreover, it is difficult to fabricate a filter with multiple windows. The solution with filter is not suitable for use in a communication system with multiple wavelengths.
A U.S. Pat. No. 5,283,686 by General Instrument Corp. disclosed an optical circulator and a fiber grating to reduce an accumulating rate of the ASE noise. The amplified optical signals and the ASE noise can simultaneously propagate to the fiber grating through the optical circulator. The fiber grating has a central wavelength, which is equal to the desired wavelength. The incident amplified optical signals are reflected by the fiber grating and are exported by the optical circulator at the other end, while the ASE noise will travel through the fiber grating. Therefore, the ASE is filtered out. The transmission quality of the optical signals remains without affection. However, the optical circulator is very expensive. It causes a great increase of fabrication cost. Moreover, the IL effect is still large. This is also not a good solution to improve the communication quality.
As previously mentioned, during the optical amplification process, the optical signals and the noise are amplified at the same time. The ASE noise can also propagate along with the amplified optical signals, resulting in a severe issue of the optical communication. The ASE noise not only reduce the efficiency, but also may cause communication error.